Chmucd Engineering Science, Vol. 42, No. 3, pp. 479491. 1987. Printe d tn G re a t Brita in. 0009-2509/87 S3.W+O.O0 0 1987. Pergamon Journals Ltd. BUBBLE PROPERTIES IN LARGE-PARTICLE FLUIDIZED BEDS L. R. GLICKSMAN, W. K. LORD+ and M. SAKAGAMIf Mechanical Engineering Department, Massachusetts InstituteofTechnotogy, Cambridge, MA 02139, U.S.A. (Received 17 July 1984, infinalform 9 July 1985) Abstract-Bubble properties were studied in a two-dimensional bed and a square bed 1.2m wide fluidized with iarge particles (1 mm average diameter). Horizontal and vertical arrays of optical probes were used to measure the bubble characteristics. The velocity of a bubble depends on the proximity of bubbles above it; the average velocity is closely approximated by the Davidson and Harrison expression. At high flow rate, the initial bubble size is closely estimated by a Froude number correlation. The rate of bubble coalescence is well- correlated by the void fraction of bubbles in the bed and can be predicted by allowing for the random spatial distribution of bubbles. A one-dimensional model for the bubble growth is presented. INTRODUCTION The hydrodynamics of a fluidized bed have a primary influence on bed characteristics such as solid and gas mixing, heat transfer to immersed surfaces and elutri- ation of particles from the bed. For beds operating in the bubbling regime, the bed hydrodynamics are largely governed by the number, size and motion of bubbles passing through the bed. There have been numerous studies of bubble charac- teristics in small-particle beds. However, the extension of these results to large-particle beds is uncertain. Davidson and Harrison (1963) developed a model for the gas flow through isolated bubbles but their model is strictly applicable only to viscous dominated gas flow through the dense phase. For air at ambient conditions, this corresponds to particles of 4OOpm diameter or less. Bubble shapes in small-particle three- dimensional beds have been observed by Rowe (1971). Whitehead and Young (1967) and Werther (1974), among others, have measured the characteristics of bubbles in large beds of small particles. They noted the presence of preferred bubble tracks in the bed cross- section where most of the bubbles were observed. Only a limited number of large-particle experiments have been performed. Cranfield and Geldart (1974) and McGrath and Streatfield (1971) measured the eruption size of bubbles at relatively low superficial velocities. The eruption size was related to bubble size by the use of relationships established for small- particle beds. Fitzgerald (1980) and Staub et al. (1980) also carried out experiments in large-particle beds. Staub’s work was concentrated on the turbulent flow regime; Fitzgerald did not report quantitative measurements of bubble properties such as size and velocity. Given the dearth of large-particle experimental data, there is no reliable means to scale up results obtained +Presentaddress: Pratt &Whitney Aircraft, East Hartford, CT 06108, U.S.A. *Present address: Sumitomo Metal Industries Ltd., Osaka, Japan. from small particles, small-scale equipment or low superficial gas velocities. The present paper is an attempt to add more information on the bubble properties in large-particle fluidized beds. Based on these observations, a model of the bubble charac- teristics will be developed. EXPERIMENTAL The experiments were performed on a column 1.17 m square. One wall was of Plexiglas to allow visual observation and video records to be made of the bed. A perforated distributor plate was used with uniformly spaced, 3.2mm diameter holes with a hole density of 2300 holes/m’. The holes backed by mesh gave a high distributor pressure drop to maintain uniform flow. At minimum fluidization, the distributor pressure drop was approximately one half the bed pressure drop. Tests were carried out both with the bed open and with the bed containing a bank of 5.1 cm diameter tubes (see Fig. 1). Tests were also carried out with the open bed in a two-dimensional configuration with a cross section of 8.3cm by 117cm. Graded silica sand with a mean diameter of 1 mm was used. Figure 2 shows the size distribution. The minimum fluidization velocity was 0.58m/s with the 0 0 0 0 0 0 0 - o-a47 000000 - 0711 ooooooo- _ 0.576 E 000000 - 0.440 0 0 0 0 0 0 0 -0305 O.l3mO.l5m 305% Fig. 1. Tube bank geometry. 479